Reference for Keritot 28:10
אם עבר זקן ונשאה חייב עליה משום אשת אב
<big><b>MISHNAH: </b></big>IF ONE HAD INTERCOURSE WITH HIS DAUGHTER'S DAUGHTER HE MAY THEREBY BECOME GUILTY FOR OFFENDING WITH HIS DAUGHTER'S DAUGHTER, HIS DAUGH TER-IN-LAW, HIS BROTHER'S WIFE, THE WIFE OF HIS FATHER'S BROTHER, HIS WIFE'S SISTER, A MARRIED WOMAN, AND FINALLY A MENSTRUANT WOMAN.<span class="x" onmousemove="('comment',' The inter-relationships between the man and his grand-daughter were manifold so that seven prohibitions were simultaneously broken in one act, viz., the grand-daughter, now a married woman, had previously wedded one of his sons and after his death the offender's brother and later, after the latter's death, the brother of the offender's father. The offender was at the same time married to his grand-daughter's half-sister, i.e., another daughter of his granddaughter's husband from another wife. The grand-daughter was, in addition, menstruant at the time of contact.');"><sup>5</sup></span>